1. Field of the Invention
The present invention relates to sulfur tolerant catalyst composites useful for reducing contaminants in exhaust gas streams, especially gaseous streams containing sulfur oxide contaminants. More specifically, the present invention is concerned with improved NOx trap catalysts for use in diesel engines as well as lean burn gasoline engines. The sulfur tolerant NOx trap catalyst composites comprise a platinum component, a support, a NOx sorbent component, and a spinel material prepared by calcining an anionic clay material represented by the formula MmNn(OH)(2m+2n)Aa.bH2O, wherein the formula is defined herein. The sulfur tolerant NOx trap catalyst composites are highly effective with a sulfur containing fuel by trapping sulfur oxide contaminants which tend to poison conventional NOx trap catalysts used to abate other pollutants in the stream. The sulfur tolerant NOx trap catalyst composites are suitable for diesel engines because the composites can be regenerated at moderate temperatures with rich pulses, rather than at high temperatures.
2. Related Art
Diesel powered vehicles represent a significant portion of the vehicle market worldwide. In Europe, the market share of diesel passenger cars is about one third and is expected to grow even higher in the years ahead. Compared to gasoline powered vehicles, diesel vehicles offer better fuel economy and engine durability. As diesel passenger cars become more popular both in Europe and elsewhere, emissions reduction is an increasingly urgent issue. In fact, Euro Stage IV regulations (year 2005) are calling for a 50% reduction of NOx emissions (0.25 g/km) compared to the Stage III (year 2000) level (0.5 g/km). For some vehicles, it would be difficult to meet the Euro IV NOx emissions target by engine improvement alone. It may be impossible to meet Euro V NOx regulations (0.125 g/k) without highly efficient after-treatment technologies.
Reducing NOx from diesel exhaust is very challenging. The 3-way catalyst technology, which is widely used in the gasoline cars, is not operational in diesel vehicles. A 3-way catalyst requires the exhaust emissions near a stoichiometric point, neither fuel rich (reducing) nor lean (oxidizing), while diesel emissions are always lean. In the early 90's, the concept of NOx trap catalyst was explored for lean burn gasoline engines where the NOx catalyst would trap NOx in a lean environment and reduce it in a rich environment.
To apply the NOx trap concept to diesel passenger cars, some special issues related to diesel emission characteristics needed to be addressed. The exhaust temperature for light-duty diesel vehicles is typically in the range of 100-400° C., which is much lower than the gasoline exhaust. Therefore, low temperature activity for oxidation and reduction is critical. One of the most difficult challenges in applying this concept is the issue of sulfur poisoning. The exhaust sulfur forms a very strong sulfate on any basic metal site, which prevents the formation of nitrate, rendering the catalyst ineffective for trapping NOx. As with other catalytic converters, thermal stability is another important issue for practical application.
The operation of a NOx trap catalyst is a collection of a series of elementary steps, and these steps are depicted below in Equations 1-5. In general, a NOx trap catalyst should exhibit both oxidation and reduction functions. In an oxidizing environment, NO is oxidized to NO2 (Equation 1), which is an important step for NOx storage. At low temperatures, this reaction is typically catalyzed by precious metals, e.g., Pt. The oxidation process does not stop here. Further oxidation of NO2 to nitrate, with incorporation of an atomic oxygen, is also a catalyzed reaction (Equation 2). There is little nitrate formation in absence of precious metal even when NO2 is used as the NOx source. The precious metal has the dual functions of oxidation and reduction. For its reduction role, Pt first catalyzes the release of NOx upon introduction of a reductant, e.g., CO (carbon monoxide) or HC (hydrocarbon) (Equation 3). This may recover some NOx storage sites but does not contribute to any reduction of NOx species. The released NOx is then further reduced to gaseous N2 in a rich environment (Equations 4 and 5). NOx release can be induced by fuel injection even in a net oxidizing environment. However, the efficient reduction of released NOx by CO requires rich conditions. A temperature surge can also trigger NOx release because base metal nitrate is less stable at higher temperatures. NOx trap catalysis is a cyclic operation. Base metal compounds are believed to undergo a carbonate/nitrate conversion, as a dominant path, during lean/rich operations. The sulfur poisoning of a NOx trap catalyst is depicted below in Equations 6-7. In Equation 6, S occupies a site for NOx and in Equation 7, SOx replaces CO3 or NOx.
Oxidation of NO to NO2NO+1/2O2→NO2  (1)NOx Storage as Nitrate2NO2+MCO1+1/2O2→M(NO3)2+CO2  (2)NOx ReleaseM(NO3)2+2CO→MCO3+NO2+NO+CO2  (3)NOx Reduction to N2NO2+CO→NO+CO2  (4)2NO→2CO→N2+2CO2  (5)SOx Poisoning ProcessSO2+1/2O2→SO3  (6)SO3+MCO3→MCO4+CO2  (7)
In Equations 2, 3, and 7, M represents a divalent base metal cation. M can also be a monovalent or trivalent metal compound in which case the equations need to be rebalanced.
A similar cyclic mechanism has been suggested based on thermodynamic calculations, where barium is chemically transformed via a carbonate to nitrate to oxide to carbonate cycle. Analyses of the gaseous products during NOx trapping experiment show that CO2 formation and NOx disappearance is exactly balanced. Since base metal oxides and hydroxides are less stable thermodynamically than their corresponding carbonates and nitrates, it is believed that the prevalent barium species in a waking NOx trap catalyst cycles between carbonate and nitrate. This, however, does not exclude the existence of other species (oxide and hydroxide) in minor quantity.
Comparative investigations on the currently most discussed lean burn DeNOx technologies comprising the continuously operating selectively catalytic reduction (SCR) of V-, Pt-, Ir-technologies as well as the discontinuously operating NOx adsorption technology suggest that the latter technology shows the most promising overall performance in terms of NOx, HC and CO removal in view of the proposed EURO III/IV legislation. The relevant operational parameters of the NOx adsorption technology are discussed (i.e. space velocity, NOx throughput, temperature and oxygen concentration) in order to outline the potential of this technology for vehicle application. Furthermore, it is demonstrated, that particularly (hose NOx storage elements, which have the widest NOx operation window on the temperature axis, unfortunately have the highest affinity for the formation of thermally stable sulfates. Consequently, poisoning by sulfur generally is an inevitable side effect of efficient NOx storage. The sulfur concentration wields decisive influence on the long-term ;activity of the NOx adsorption catalysts and it is shown by a worst case study, that even the use of low-sulfur fuel does not need to prevent the accumulation of sulfur on the NOx adsorption catalyst. The accumulation of sulfur on the catalyst has to be counteracted by an engine induced desulfation strategy, by which the sulfur is driven out of the NOx adsorption catalyst. This requires the provision of reducing exhaust gas at elevated temperature for a short period of time. An optimization of the desulfation parameters is mandatory in order to suppress the formation of H2S. It is conjectured that the thermal degradation of the NOx adsorption catalyst proceeds via two different deactivation modes. The first one is based upon the loss of Pt dispersion and is accelerated by the presence of oxygen while the second one can be traced back to the reaction between NOx storage components and the porous support material. Wolfgang Strehlau et al., Conference “Engine and Environment” 97.
Direct injection technology for diesel engines as well as for gasoline engines are the most favored ways to reduce the CO2 emissions in the future. NOx adsorber technology for gasoline DI engines as well as for HSDI diesel engines is the favored technology to meet future emission limits. Adsorber catalysts have demonstrated their potential to meet future emission legislation levels on prototype basis for gasoline and diesel engines. Improving the NOx adsorber technology and the integration of the adsorber system into the power-train system for the introduction into the European market is the challenge for the near future.
U.S. Pat. No. 5,407,652 (Swamy et al.) discloses a process for removing N2O from an N2O containing gaseous mixture. The process comprises heat treating a catalyst precursor to form a catalyst and reacting the N2O containing gaseous mixture in the presence of the catalyst under conditions sufficient to convert the N2O to gaseous nitrogen and gaseous oxygen. The catalyst precursor comprises an anionic clay material represented by the formula, MmNn(OH)(2m+2n)Aa.bH2O, wherein M is a divalent metal cation; N is a trivalent metal cation; A is a mono-, di-, or trivalent anion which decomposes when heated to a temperature sufficient to form a volatile gas; m and n are such that m/n has values of 0.5 to about 6; a is a number with the proviso that when A is a monovalent anion, a=n, when A is a divalent anion, a=1/2n and when A is a trivalent anion a=1/3n; and b is an integer having values of 1 to 10. The anionic clay material is heated to a temperature sufficient to cause A, the mono-, di- or trivalent anion, to decompose to form a volatile gas.
U.S. Pat. No. 5,727,385 (Hepburn '385) discloses a catalyst system, located in the exhaust gas passage of a lean-burn internal combustion engine, useful for converting carbon monoxide, nitrogen oxides, and hydrocarbons present in the exhaust gas. The catalyst system comprises two-components: (1) a lean-burn nitrogen oxide catalyst being a transition metal selected from the group consisting of copper, chromium, iron, cobalt, nickel, iridium, cadmium, silver, gold, platinum, manganese, and mixtures thereof loaded on a refractory oxide or exchanged into zeolite; and (2) a nitrogen oxide (NOx) trap material which absorbs NOx when the exhaust gas flowing into the trap material is lean and releases the absorbed NOx when the concentration of oxygen in the inflowing exhaust gas is lowered. The nitrogen oxide trap material is located downstream of the lean-burn nitrogen oxide catalyst in the exhaust gas passage such that the exhaust gases are exposed to the lean-burn catalyst prior to being exposed to the nitrogen oxide trap material.
U.S. Pat. No. 5,750,082 (Hepburn et al. '082) discloses a nitrogen oxide trap useful for trapping nitrogen oxide present in the exhaust gases generated during lean-burn operation of an internal combustion engine. The trap comprises distinct catalyst phases: (a) a porous support loaded with catalyst comprising 0.1 to 5 weight % platinum; and (b) another porous support loaded with 2 to 30 weight % catalyst of an alkaline metal material selected from the group consisting of alkali metal elements and alkaline earth elements.
U.S. Pat. No. 5,753,192 (Dobson et al.) discloses a nitrogen oxide trap useful for trapping nitrogen oxide present in an exhaust gas stream generated during lean-burn operation of an internal combustion engine and releasing the absorbed nitrogen oxides when the oxygen concentration of the exhaust gas is lowered. The trap comprises a porous support loaded with 6-15 wt. % strontium oxide; and loaded thereon together: (a) 0.5-5 wt. % precious metal selected from platinum, palladium, rhodium and mixtures thereof; (b) 3.5-15 wt. % zirconium; and (c) 15-30 wt. % sulfate.
U.S. Pat. No. 5,758,489 (Hepburn et al. '489) discloses a nitrogen oxide trap useful for trapping nitrogen oxide present in the exhaust gases generated during lean burn operation of an internal combustion engine. The trap comprises a porous support; and catalysts comprising at least 10 weight percent lithium and 0.2 to 4 weight percent platinum loaded on the porous support.
U.S. Pat. No. 5,759,553 (Lott et al.) discloses a NOx adsorber material comprising an activated alkali metal-doped and copper-doped hydrous zirconium oxide material that adsorbs NOx in an oxidizing atmosphere and desorbs NOx in a non-oxidizing atmosphere.
U.S. Pat. No. 5,910,097 (Boegner et al.) discloses an exhaust emission control system for an internal combustion engine. The system comprises two adsorber parts arranged in parallel for alternate adsorption and desorption of nitrogen oxides contained in an exhaust from an engine. A means for conducting the exhaust further downstream is provided emerging from one of the two adsorber parts currently operated in the adsorption mode and for recycling the exhaust emerging from the other of the two adsorber parts operating in the desorption mode into an intake line of the engine. An oxidizing converter is located near the engine and upstream from the adsorber parts for oxidation of at least NO contained in the exhaust to NO2. An exhaust line section is located upstream of the adsorbs parts and is divided into a main line branch and a partial line branch parallel to the main fine branch. The two adsorber parts are connected by control valves to the main line branch and the partial line branch such that The one adsorber part that is operating in the adsorption mode is fed by the exhaust stream from the main line branch and the other adsorber part that is operating in the desorption mode is supplied by the exhaust stream from the partial line branch.
European patent applications 589,393A2 discloses a method for purifying an oxygen rich exhaust gas by simultaneously removing the carbon monoxide, hydrocarbons, and nitrogen oxides contained in the exhaust gas. The method comprises bringing the oxygen rich exhaust gas into contact with an exhaust gas purifying catalyst comprised of (i) at least one noble metal selected from the group consisting of platinum and palladium (ii) barium, and (iii) at least one metal selected from the group consisting of alkali metals, iron, nickel, cobalt and magnesium, supported on a carrier composed of a porous substance.
European parent application 669,157A1 discloses a catalyst for purifying exhaust gases. The catalyst comprises a heat resistant support; a porous layer coated on the heat resistant support; a noble metal catalyst ingredient loaded on the porous layer; and an NOx storage component selected from the group consisting of alkaline-earth metals, rare-earth elements and alkali metals, and loaded on the porous layer. The noble metal catalyst ingredient and the NOx storage component are disposed adjacent to each other, and dispersed uniformly in the porous layer.
European patent application 764,459A2 discloses a nitrogen oxide trap useful for trapping nitrogen oxide present in the exhaust gases generated during lean-burn operation of an internal combustion engine. The Imp comprises distinct catalyst phases (a) a first porous support loaded with catalyst comprising 0.1 to 5 weight % platinum; and (b) a second porous support loaded with 2 to 30 weight % catalyst of a material selected from the group consisting of alkali metal elements and alkaline earth elements.
European patent application 764,460A2 discloses a nitrogen oxide trap useful for trapping nitrogen oxide present in the exhaust gases generated during lean-burn operation of an internal combustion engine. The trap comprises a porous support; and catalysts consisting of manganese and potassium loaded on the porous support.
Laboratory and engine tests were carried out to describe the sulfur effect on the NOx adsorbers catalysts efficiency for gasoline lean burn engines. One aspect of the study dealt with the NOx storage efficiency of the adsorber under laboratory conditions, especially regarding the SO2 gas phase concentration. The rate of sulfur storing is greatly affected by the SO2 gas concentration. While 6.5 hours are required to get from 70% NOx reduction to only 35% when the gas mixture contains 10 ppm SOx, it takes 20 hours with 5 ppm of SOx and more than 60 hours with the 2 ppm SO2 condition. The relationship between the loss in NOx trap performance and SO2 concentration appears to have an exponential shape. The same amount of sulfur (0.8% mass) is deposited onto the catalyst within 10 hours with the feed gas containing 10 ppm of SO2 and within 50 hours with 2 ppm SO2. Nevertheless, it was shown that the loss in NOx-trap efficiency is not the same in these two cases. The efficiency decreased from 70% to 25% in the first case (with 10 ppm SO2) and from 70% to only 38% in the second case (with 2 ppm SO2). The second aspect describes a parametric study on engine bench concerning the sulfur effect on NOx trap efficiency and the required conditions (temperature, air/fuel ratio) to obtain different rates of desulfation. For instance, after 70 hours, NOx efficiency decreased from 90% to 25% with a sulfur content in gasoline of 110 ppm. Complete regeneration requires various durations of desulfation depending on air/fuel ratio (gamma=1 to 0.95) and temperature conditions (950 to 750° C.). For example, complete regeneration occurs after several minutes at gamma=1 and several sets of ten seconds at gamma=0.95 at 650° C. Results show that sulfur content close to EURO III gasoline standards is the main obstacle for the introduction of NOx absorber catalyst in Europe. Impact of Sulfur on Nox Trap Catalyst Activity Study of the Regeneration Conditions, M. Guyon et al., Society of Automotive Engineers, 982607 (1998).
The conventional catalysts described above employing NOx storage components have the disadvantage under practical applications of suffering from long-term activity loss because of SOx poisoning of the NO traps. The NOx trap components employed in the catalysts tend to trap SOx and form very stable sulfates which require regeneration at 650° C. which is not practical for low temperature diesel exhaust. Accordingly, it is a continuing goal to develop NOx trap catalysts which can reversibly trap SOx present in the gaseous stream and thereby prevent SOx sulfur oxide poisoning of the NOx trap and an he regenerated at moderate temperatures with rich pulses, rather than at high temperatures.